Why distance protection is needed for transmission lines?

The power transmission from the power generation station is achieved with the help of a transmission line so proper maintenance of the transmission lines should be done. The distance protection would do the protection of the transmission lines and also the sub-transmission lines. In case if there is any fault in the transmission line then it would lead to loss of load and other problems too. The major need for transmission line protection is that the line faults must be cleared selectively and at high speed. The distance protection is done by using a distance relay, this relay doesn’t compare the local line current to the current at the far end of the line. The distance relay would compare the local current, with the local voltage in the corresponding phase

What are the major features of distance protection?

  • Good accuracy because more information is used to make a decision
  • This type of protection is directional it would respond to the phase angle of current with respect to voltage phasor
  • Accurate and fast
  • Selectivity and sensitivity
  • Back up protection
  • It is mostly used for the protection of the transmission lines and it can also be used for the transformer protection
  • The tripping distance can be determined with the local measured quality and also can respond to the external faults by providing a time-delayed backup protection function
  • Distance protection is used for the protection of extra high voltage transmission and sub-transmission
  • The major advantage of distance protection is that it would do the fault coverage of the circuit independent of the variations in the source voltage
  • It would provide primary and backup protection

What are the various measuring characteristics of distance relay?

  • Impedance characteristics
  • Mho characteristics
  • Reactance characteristics
  • Linear characteristics
  • Lenticular characteristics
  • Quadrilateral characteristics

What are the advantages of distance relay over the over-current relay?

The overcurrent relay can’t be used for distance protection because it has a disadvantage of variable reach and also the variable operating time because of the variation in the source impedance and also the type of fault. The operation of the distance relay is different from the overcurrent relay it would operate when there is impedance, the impedance could be because of the components. So this impedance will be compared to a preset value. The reach of the relay is the corresponding distance or the preset impedance. The distance relay can provide better protection than the overcurrent relay, basically, it can overcome the problems related to the overcurrent relay.

What are the required parameters in order to set the distance protection?

Line parameters

  • Length of the line, size of the conductor voltage level, and also the conductor type
  • The above-mentioned details are used to calculate certain parameters such as reactance, positive, and zero sequence resistance.
  • Maximum load current 
  • Number of conductors

Transformer parameters

  • Positive and zero sequence test value
  • Voltage ratio
  • MVA rating
  • Impedance value percentage

Current and voltage transformer ratios

  • The ratio of the PT and the CT must be provided

Arc & tower footing resistance

  • It is used in resistive reach calculation

How does a distance relay work?

This type of relay is a double-acting relay in which one coil would be energized by voltage and the other one will be energized by the current. So the torque is produced in a way that when the V/I reduces below a set value then the relay would operate. In case if there is any fault in the transmission line then the fault current would increase and the voltage at the fault point will be reduced. So then the ratio V/I will be measured at the location of the CTs and VT’s. The voltage which is at the VT location is dependent on the distance between the VT and the fault. If the fault is closer to the relay then the measured voltage will be low, and if the fault is not near to the relay then the measured voltage will be high.

The transmission line impedance is proportional to its length so we can use the distance relay and it would operate for the faults that occur in between the relay location and the predetermined point. Then the calculated impedance is compared with the setpoint impedance and if the measured impedance is less than the setpoint impedance then there is a fault between the relay and the set point.

What are the different zones in distance relay protection?

Distance relay mostly has three zones of protection

  • Instantaneous zone (1st zone)
  • Time-delayed and back up zone (3rd and 4th zone)

These zones are created so that the reliability of the system can be improved. The primary line is protected by zone1. This zone provides quick protection because there is no intentional time delay. Zone one protects eighty percent of the primary line.

The second zone would allow the 1st zone to clear the faults, zone two would handle the hundred percent of the protected line and also the fifty percent of the shortest line which is connected to the station. Zone3 would act as backup protection for zone2. This zone can perform as remote protection or backup for the relay and it would also handle the station failures.

What are the types of distance relays?


This is a voltage restrained overcurrent relay, this relay would measure the line impedance by using the voltage and the current which is applied to the relay. So during any fault, the current would rise and the voltage will decrease. This relay would detect the impedance according to the current and voltage. The relay would determine the impedance if it is in the reach setting of the relay. The impedance of the line will be proportional to the fault distance and that’s why it is called distance protection. This type of distance relay is for the medium-length transmission line.

Disadvantages of impedance relay

Mostly the fault in the transmission line is arc and its nature is resistive and the fault characteristics will be modified and this would affect the operation of the impedance relay.


In this relay, the operation torque is received from the current and the restraining torque because of the directional element of the current and voltage. So we can consider the reactance relay as an overcurrent relay with directional restraint. This relay would operate only if the measured reactance is lower than the set value. The resistive component of the impedance won’t affect this relay, this relay would only react to the reactance of the impedance. This is a non-directional relay and it can be used to detect the earth faults in which the arc resistance will be high.


This type of relay can be used for long-distance transmission line protection and these are directional relays. In order to convert the impedance relay to a directional relay, a separate unit is needed and it can’t be used with the reactance relay. In the Mho relay, a voltage winding is added and this is called the polarized winding to make it directional. This relay operates by measuring the admittance of the operating torque and restraining torque are received from the voltage and current. This relay would operate if the impedance is within its protected region. The Mho relays protected region can be considered as a circle and this circle is composed of the impedance maximum reach, maximum torque angle, and also the relay characteristics angle.

What are the advantages of distance relays?

  • Good instantaneous trip coverage with security
  • The impedance zone has a fixed impedance rich
  • It is easy to set calculations and co-ordinations
  • Protection will be in a fixed zone which is relatively impedance of the system change
  • There is no need for readjustment because it has a permanent setting

What are the major factors that could affect the operation of the distance relay?

  • Fault resistance
  • Infeed effect
  • Branching off effect
  • Load encroachment

What are the major applications of distance protection?

  • Protection of transformer
  • Protection of generator
  • Feeder protection (Transmission line) by pilot wire differential protection
  • Big motor protection
  • Protection of bus zone
  • Transmission line protection by phase comparison carrier current protection

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